That statement only applies to GNU/Linux systems, or at least systems
that use the GNU Binutils. Most commercial UNIX vendors don't have
long option support in their Binutils. Sun and HP's versions of tar don't
support long arguments, but FreeBSD and Linux support both forms of
arguments.

While the choice of which arguments your programs accept is your choice,
many people choose to follow the standard their OS vendor uses.
I think that the table showing only the short arguments is good
because short arguments is more of a standard then GNU-Style arguments,
but I think that including a link to GNU Coding Standards Option Table,
with an explaination that these are used mainly on GNU/Linux or GNU Binutils
systems would be useful.